Seismic surveying



et. 2%, 194 6. 0. s. PETTY SEISMIC SURVEYING '2 Sheets Sheet 1 OriginalFiled .Aug. 18, 1939 Patented Oct. 29, 1946 SEISMIC SURVEYING Olive S.Petty, San Antonio, Tex.

Continuation of application Serial No. 290,928, August 18, 1939. Thisapplication May 27, 1943, Serial No. 4885764 4 Claims.

portions of the waves are amplified to usable levels. The inventioncontemplates especially certain improvements in methods and apparatusfor amplifying and recording the waves and the application is acontinuation of my prior application Serial No. 290,928, filed August18, 1939.

Heretofore in making use of seismic methods and apparatus for surveyingsub-surface geological formations, a great many difliculties have arisenbecause of the limitations of -equipment available, and it has notalways been possible to make an accurate study of the records or chartsproduced because of the intermingling of several types of waves whoserelative sizes may be on the order of 600 to 1.

To understand some of these difliculties a review of the general methodsof use of seismic recording apparatus may be briefly made at this point.Such surveys are made by correlatingthe results attained from aplurality of records each obtained by generating seismic waves, forinstance by firing a charge of dynamite below the surface of the earthat a position generally referred to as the shot point, and detecting, atremote points, the time of arrival of radiating seismic Waves as well asthe instant of the explosion and recording the same on a seismogram,usually by photographic methods.

The seismic waves generated in the earth by the exploding charge travelin all directions and some arrive at the receiving or detectingstations, at remote points, by short paths of travel through strata nearthe surface of the earth. These are known as direct waves and because oftheir relatively short path and major horizontal component of vibration,usually arrive first .at the detector. A second form of waves referredto as reflected, travel down to horizons at various distances below thesurface of the earth and at various inclinations thereto and arereflected back to the receiver. Such waves from the upper horizonsusually travel in longer paths than the direct waves, while those fromthe deeper horizons always travel in longer paths and take more time toreach the detector than do the direct waves. The reflected wavesconsisting largely of vertical components have an intensity usually 2very much less thanthat of the direct waves when travelling from upperhorizons, while those from progressively deeper horizons are more andmore attenuated, having sometimes only /600 of the intensityof thedirect waves.

It is-customary to record on a single chart .the record traces from aplurality of detectors and it is highly desirable to keep vthe physicalsize of the chart within reason. The waves are converted from seismic toelectrical at the detector, but there still remains the tremendousdiscrepancy in peak amplitudes between the direct and the reflectedwaves as Well as the problem of attenuation of the later arrivingreflected waves. Most detectors seek to convert the seismic waves toelectro-motive-forces, the voltages of which are indicative of theamplitudes of the corresponding seismic waves.

The outputs ,from .such detectors are usually at such low levels thatthey are not adequate throughout the whole wave train to operate therecorder which includes a galvanometer having multiple moving elementseach responsive to the voltages of the waves from one detector. Resortis therefore had to electric .am.. plification to raise the level of thewaves so that the weakest produce a usable trace on the chart. If thisis done with the conventional amplifier having uniform gain throughout,thetraces of the direct waves become so large that they cannot be keptwithin the physical confines of the chart and furthermore those onadjoining traces tangle on the record. Likewise there is difiiculty ofentanglement of .the moving systems of a multiple galvanometer therebyvitiating the record and probably damaging the instrument. withexcellent damping applied to the moving systems of the galvanometer, thetremendous swings imparted by the oversized direct waves prevent rapidsubsidence and accurate recording of the .earlier portions of thereflected waves, and it is highly important tojknow the instant thefirst reflected wave is received. It is also important to know the.instant of arrival of the first direct wave.

To overcome the various difliculties outlined above, the presentinvention contemplates the provision of an amplifier, to be insertedintermediate the detector and the galvanometer of the recorder, whichhas such characteristics as 'to substantially eliminate the diflicultiesand to provide on the chart a trace of the seismic waves having ageneral average level of maximum swings which is substantially uniform.or increases toward the end of the trace.

.To obtain these improved results it is an 0b- Even ject of the presentinvention to provide an amplifier for use between a seismic detector andrecorder which is provided with an automatic gain adjuster at all timesunder the control of the wave size.

Another object of the invention consists in the arrangement of anamplifier whereby it produces at all times such gain as tends to levelout the general average of the maximum swing heights of the responsecurve of the detector.

An important feature of the invention comprises a gain controlresponsive to the output of the amplifier in which the gain remainsconstant until the output passes a predetermined level after which thegain is maintained at such value as to substantially hold this levelduring the tendency to excess output and thereafter slowly returns tonormal over a period of several seconds whereby compensation forattenuation at the extreme end of the wave train is made.

Another important feature of the invention comprises an amplifier gaincontrol operating by superimposing portions of the plate current of anautomatic volume control tube onto the grid of the amplifier tube ortubes and in using a condenser charged from this plate current todeliver the same over a period of time to said grid, whereby no abruptincrease in gain is effected after any decreases.

Still another important feature comprises apparatus for regulating therate of discharge of said gain control condenser, the time over whichthe discharge takes place, and the type of discharge-time curveresulting.

A further important feature of the invention comprises means fordetermining the threshold value below which the gain normally remainsconstant and above which any increase in output causes a correspondingdecrease in gain. This threshold value may itself remain fixed or mayvary throughout the taking of a record in any predetermined and desiredmanner. In general, I prefer that the threshold value shall not befixed, as regards the amplitude of energy sufiicient to initiatefunctioning of the gain control, but that it shall vary in accordancewith the frequency of the received signals.

For example, I have found that it is desirable to maintain apredetermined average amplifier output for the highest usablefrequencies, and a somewhat larger average output of the amplifier whenthe energy is of lower frequencies. This would preferably varycontinuously from the highest to the lowest usable frequency. Thus wherehigh frequencies predominate the peaks of the recorded traces are quitesharp and a large amplitude is not required for readily locating thepeak. If the same amplitude is used whenever the low frequenciespredominate, on the contrary, the peaks are not sharp and are difficultto readily locate. This difiiculty can be overcome by using a largeramplitude for the low frequencies. If a largenumber of traces are placedon a single record this would mean that one trace would deviate beyondthe rest position of one or more of the traces near it. This is notobjectionable for low frequencies since it is much easier to follow thetraces when they deviate slowly. The low frequency energy is usuallyfrom the deep reflecting beds, so there is very little step-out from onetrace to the next trace; in other words, each trace nearly duplicatesthe trace above. it where low frequencies exist. This means that thissimilarity of adjacent traces permits the use of appreciably largeramplitude without the overlapping of the traces. If this same largeamplitude were used for the high frequencies, it would be difficult tofollow them since they would be moving so rapidly. It would also bedifficult to follow the high frequency energy at large amplitudesbecause there would not be the duplication of the traces from one to thenext since the normal step-out of the reflected energy from shallow beds(which produces high frequency energy) may be a half a cycle or more; inother words, instead of the traces being merely identical, one above theother, they will be shifted by one-half cycle or more. For this reasonit is extremely desirable that the traces not overlap when highfrequency energy is being recorded. It will readily be noticed that theinstant application shows a method whereby all of the above advantagesare obtained. Whenever the low frequencies predominate, on the contrary,very high gain is helpful in emphasizing the peaks of the low frequencycycles and there is no confusion in the record since the separate cyclesare more widely spaced.

It is therefore an object of the invention to provide automatic gaincontrol which tends to suppress the amplification of higher frequenciesand to favor the amplitude of lower frequencies within a usablefrequency range. Ordinarily the effect of such an arrangement inpractice is to.

establish increased amplitude or sensitivity toward the end of therecord since seismic waves reflected from the deeper beds and arrivinglater usually tend to be lower in frequency as well as in amplitude.Consequently, in an amplifier in which higher frequencies are relativelymore effective in reducing the gain, the automatic gain control is,generally speaking, more effective near the beginning of the record thantoward the end of the record.

Other and further features and objects of the invention including theprovision of various filters and the like for improving the operation ofthe whole amplifier will be more apparent to those skilled in the artupon a consideration of the accompanying drawings and followingspecification wherein are disclosed two exemplary embodiments of theinvention together with various curves representing the operation ofimportant portions of the circuits.

In said drawings:

Figure 1 is a wiring diagram of an amplifier constructed according tothe present invention shown connected to the output of a detector;

Figure 1A is a fragmentary view of a modified form of the last stage ofamplification; I

Figure 2 is a curve showing the amplifier gain at various frequencieswithout the use of a filter;

Figure 3 is a similar curve showing the gain with a filter in circuit;

Figure 4 shows similar curves with possible filter adjustments;

Figure 5 is a characteristic curve of the automatic volume control tubeshowing how the grid bias can be adjusted to cut off plate current;

Figure 6 is a curve showing how the grid potential of the A. V. C. tubeis modified by the amplifier output;

Figure '7 is a corresponding curve illustrating the resultant platecurrent fiow;

Figure 8 is a set of curves showing various possibilities for changingthe rate, time, and style of current discharge from the gain controlcondenser; while Figure 9 shows the resultant amplifier gain; and

Figure is a set of curves showing the effect of the filter resistancevalue on the time of current leak off.

Referring now to the drawings for a better understanding of an actualembodiment of the invention, there is shown in Figure 1 a detector orseisometer I5 of any suitable type adapted to convert seismic waves towave-form electrical energy. The detector shown happens to be equippedwith a single stage pre-amplifier and therefore the cable leading to theamplifier of the present invention includes four conductors I5, 11, i8,and IS, the last two of which conduct filament heating current from asuitable A battery as represented at the left-hand side of the figure.The conductors l6 and I? carry the 'B-supply from battery and return theoutput of the single stage amplifier to the amplifier of the invention.Each of these signal leads contains a radio frequency choke coil 2| andone of these is connected to the A conductor by means of a condenser 22for filtering out alternating currents which may be picked up by theconductors leading to the detector. These conductors are sometimesseveral hundred yards in length and may, by inductance, pick up hum fromA. C. power lines and the like. A milliameter shown at 23 indicates theplate current of the tube in the detector while the variable resistance2 5 permits regulation of the detector filament current as indicated bythe milliameter 25. This is essential because of the diiferent lengthsof conductor cable which might be used.

It is not uncommon to employ groups of detectors or seismometers inwhich the outputs of i the seismometers in each group are combined, theseismometers in the group being so spaced asto minimize the efiect ofground roll. It will be appreciated that such groups of seismometersact, for the purpose of the instant invention, as separate units and itwill be understood that whenever one seismometer or detector is referredto hereinafter, such groups having combined outputs are considered asembraced within such reference.

An incoming signal passes through the primary 25 of a suitabletransformer, the secondary '2'? of which is shunted by a pair ofvariable resistors 28 and 29, one for coarse and one for fineadjustment. These controls are for manually setting the level of thesignal applied to the amplifier of the present invention. They merelywaste a portion of the signal from a detector and hence control themaximum gain of the amplifier. They are set at one value for anyonerecord and remain unchanged during the taking of the same but are oftenchanged from one record to the next.

The secondary 2'! feeds into the cathode and grid in the right-handportion of the thermionic tube 30 which constitutes a simple triodeamplifier having the plate 3|, grid 32, and heated cathods 33 which itshares in common with the elements of the second stage amplifierindicated at 34. A suitable biasing battery 35 is arranged in the gridcircuit of the triode amplifier. This amplifier is coupled to the secondstage amplifier by means of a plate coupling resistor 36 andcondenser'3'l. The plate 3|. as well'as the plate 38 of the second stagetube receive their B battery supply through the common conductor 39connected to the positive end of the battery 40. 'A tap 4| in thisbattery supplies the screen grid 42 of the second stage amplifier withla somewhat iower potcr itial, permitting more gain'and bet .terautomatic volume control.

43 of the second stage amplifier is coupled by the The control gridcondenser 37 to the plate of the first stage and receives its biasthrough resistor 45 from a tap on battery 46.

The constants of the coupling condenser 31 from the first stage to thesecond stage, of the grid coupling resistor 45 of the second stage, andofthe plate coupling resistor 35 of the first stage are allpredetermined or adjusted to give a selective frequency response asdesired. The incoming signal in most cases is of a frequency varyingfrom 20 to 100 cycles, whereas a number of unwanted noises are at higheror lower frequencies and may thus be partially or completely eliminated.To provide a further filter action, an adjustable coupling condensergenerally denoted 4'! provides the coupling between the second stageplate 38 and the control grid 48 of the third stage amplifier valve 49,functioning along with the plate resistor 50. As shown, condenser 41comprises a plurality of small condensers, any number of which may beplaced in circuit by appropriate connections or links. A switch 5!permits removing the condenser bank from the circuit by shunting thesame. This condenser acts in the nature of a filter, first to keep theplate current of the tube from flowing through the primary winding 52 ofthe interstage coupling transformer 53, thus improving the transformerlife and permitting its design to be more eflicient. In addition, thecondenser is used to vary the frequency response of the amplifier,giving it the highest gain on the desired fre quencies and very low gainon undesired frequencies, which efiect comes from resonating thetransformer primary.

Figure 2 shows the result of operating the amplifier with the filter cutout, and it will be seen that the frequency response for the mainportion thereof has a fairly uniform gain over a wide range. Figure 3 isa similar View with the filter cut in, showing in solid line 54 a fiattopped, sharply cut off response over a short frequency range. The sizeof the plate coupling resistor may be'adjusted to give this response theflat top and much greater sharpness of resonance as shown in dotted line55 in Figure 3. Figure 4 shows how the filters may be adjusted tomaintain the same sharpness of resonance within various frequencyranges.

The secondary winding 56 of the interstage transformer 53 connectsbetween the control grid G8 and the bias battery 46. The screen grid ofthis tube is connected by wire 51 to the conductor supplying B batteryto the plates 3| and 38, although it might be connected to conductor 4|supplying potential to the screen grid of the second stage amplifier.The output from the third stage is fed through the primary 58 oftransformer 59, one secondary 60 of which delivers the amplified wavesto the moving element of a galvanometer associated with and forming partof a recorder 6 l. Another secondary 62 takes off a portion of theoutput of the amplifier for supply to the automatic volume control tubeas will 'be further described hereinafter. The condenser 63 between theplate 64 of the third stage tube and ground serves to by-pass high frequencies and to resonate the output transformer to assist in the desiredfilter action. At the same time, this condenser performs the importantfunction of assisting in the damping of the moving element of thegalvanometer. v

In Figure 1A is shown an optional way of connecting the third stage tube49 to the output transformer 59'. A second transformer 59 has a primaryin series with that of 59 and which is fed from the plate 64' of thelast amplifier stage by means of the resistance 65 and condenser 66,thus giving a resistance coupling in which the values of the condenserand resistance are critical, again assisting in frequency selection,this time, however, to determine those frequencies on which theautomatic volume control will function. Hereinafter the expressionautomatic volume control will be designated by the characters A. V. C.for convenience. The secondary of transformer 59 feeds the galvanometer.The features of Figure 1A can be substituted in Figure 1 where desiredwithout any other changes.

The second half of tube 49 is a triode containing plate 67, grid 68, andthe common heated cathode 69 which functions also with the elements ofthe third stage amplifier. The plate of this tube is connected by wire10 to a separate B battery H, the negative pole of which leads throughvariable resistors 12, 13, and 14 back to wire 15 connected to thenegative A battery and common ground terminal. It will be noted thatbias battery 46 is connected by wire I to the slider of the resistor 13and hence is also connected to the ground through resistor Hi shunted bycondenser 93. The battery H may have a potential of about 45 volts forthe type tube shown, while resistors l2, l3, and M are of the order of200,000 ohms each.

The grid 68 of the A. V. C. tube is fed from the secondary 62 of theoutput transformer 59, the opposite end of which is connected byconductor ll to the slider of a potentiometer l8 energized from theportion of B battery 40 between negative terminal 19 and adjustable tap80, through the contacts 8| of a relay, the winding 82 of which is inseries with the several tube heaters. This ensures the contacts 8|remain ing open until the amplifier is turned on, thereby not drainingthe lower portion of the B battery through the potentiometer. The mainportion of the B battery is shunted by filter condenser 83.

The slider on the potentiometer is set to supply a sufficiently negativebias to the control grid 38 of the A. V. C. tube to normally prevent theflow of any plate current in that tube. A reference to Figure 5 willmake this clear, where the characteristic curve of the tube is shown asbetween grid voltage and plate current. The cutoff point is at 86 whenthe grid is to that extent negative. The grid, however, is carriednegative to the extent indicated, for instance, at 81 and will have tobecome positive by an amount indicated by the distance between 86 and8!- before any current will flow in the plate circuit. The position of81 is adjustable by moving the slider on the potentiometer 18.

Referring now to Figure 6 the operation of the grid bias in preventingand permitting current flow in the plate circuit is illustrated. Thesinuous wave represents the output from the last amplifier stage asapplied to the grid 68 in series with its negative bias. It functions,of course, on both sides or in other words is additive to or subtractivefrom the bias setting, again indicated in this figure by 87. The amountof bias required for cut-off is shown at 85. Whenever the positivepotential supplied from the transformer winding 62 exceeds thedifference between 86 and 81 as shown in the three swings 88', the gridis forced to a sufficiently reduced bias to permit-plate current flow asindicated in Figure 7, where milliamperes of plate current are indicatedby the ordinates. The curves 89 in solid lines indicate thecorresponding plate current flow for the three swings 88 of Figure 6which cross the position of cut-off bias.

The curves of Figures 6 and 7 indicate, in general, the manner ofcontrolling the gain of the amplifier in accordance with its output for,as will later be described in more detail, the plate current from theautomatic volume control tube determines the total amount'of bias on thecontrol grids of the second and third stage amplifiers, functioning inconjunction with battery 46.

With the automatic volume control tube inoperative, i. e. wheninsufiicient output from the last stage of the amplifier is available toreduce the tube bias to permit plate current fiow, the gain of theamplifier is fixed and constant irrespective of the size of the outputas long as it remains beneath the value predetermined by the setting ofthe potentiometer 18. This setting determines the threshold value abovewhich any output from the amplifier tends automatically to reduce thegain thereof. The amplifier may be said to have a variable gain, thevalue of which is fixed until the output crosses the threshold afterwhich the gain is reduced in accordance with the tendency to increasethe output. This has a levelizing effect on the maximum amplitude ofvoltage swings delivered from the amplifier without, however, changingthe relative size of the swings beneath the threshold value except ashereinafter described.

The amount of plate current flowing to the automatic volume control tubeis determined not alone by the setting of the bias resistor 18 but bythe setting of the resistor 12 which is directly in the plate circuit.This resistor may be set, for instance, to obtain plate current swingsof the size indicated at 89 in Figure '7 or to obtain smaller swings asare indicated by dotted lines 90. The settings of the resistors 13 andM, in series with 72 and ground, are not so effective in regulating theplate current since they are shunted by fixed condenser 9! ofconsiderable capacity, for example of the order of 1 mf., whichby-passes the A. C. peak impulses to ground and at the same time ischarged with a substantially pure direct current voltage. The upperplate of condenser 9| is grounded and the potential of the lower plateis negative, feeding through conductor 92 and the portion of theresistor 13 set by means of the slider to the conductor 18 and into thepositive side of battery 46, increasing the negative. potential suppliedto the grids of the second and third amplifier tubes, thereby reducingthe gain of these amplifiers in accordance with the size of the currentswings on the A. V. C. plate.

Since the charging current for the condenser 9| consists of pulsating D.C. it is not sufficiently free from ripples to be fed directly back intothe grids of the amplifier stages. It is the function of the resistor 13and the condenser 93 to form a filter smoothing out these ripples andcausing substantially pure direct current to be applied to the grids.The setting of the resistor T3 and the size of the condenser 93 affectthe manner in which the charge on the condenser Bl leaks off to thegrids. As hereinbefore mentioned, condenser 9| may well have a capacityof the order of 1 mi. If these filter elements were not present, thecharge would leak off from condenser 8! to the grids as shown by thecurve H10 in Figure 8. Appropriate settings of the con;

g denser and resistor permit the current to leak 01f. in the mannerindicated by such curves 'as I01 and I2, increasing the time ofdischarge and changing the slope thereof in accordance with certain.characteristics desired to be accentuated on the record chart. Theresultant effect on the gain of the amplifier is indicated in Figure 9where the curve 103 indicates the operation Without the filter elements13, 93, while curves I 04 and H correspond to the condenser and resistorsettings indicated by curves IUI and I02 respectively.

The variable resistor 14 afiects the rate of leakage from the condenser93 and indirectly from condenser SI and various settings thereof resultin curves such as shown at I06, I01, and H18 in Figure 10. This resistor74 is also eifective to select the total voltage available for return tothe main amplifier tube control grids. If resistance of resistor 14 isequal to resistance of resistor 13, approximately 50% of total voltagedeveloped across M is fed back into lead 16 to reduce the gain of theamplifier. However, if resistance of 14 is four times resistance of 13,approximately of total voltage of condenser 9| will be fed back intolead 16 to vary the gain of amplifier, hence there will be a largevariation in gain for a slight excess in signal level.

Operation of the automatic gain control is particularly effective forseismic recording because the customary output from the detectorcomprises, first, waves of large amplitude directly received; second,waves of much less amplitude reflected from sub-strata. These secondwaves die out over a period of several seconds and it is highlydesirable that their amplitudes be maintained at usable size to the endof the chart. The setting of the resistor determining the bias on the A.V, C. tube should be such that each of the swings of the direct wavevoltages causes the A. V. C. tube to function and therefore thecondenser 9| is maintained charged until the last of the direct wavesceases. The first of the reflected waves is normally too small to causefunctioning of the A. V. C. tube but the gain of the amplifier does notimmediately return to normal since the charge on condenser 9| leaks offslowly and hence holds the amplifier grids at excess bias. The timerequired for this charge to leak off can be set by appropriateadjustment to extend substantially for the time during which it isdesired to record the reflected Waves, the gain slowly increasing as thestrength of the waves is reduced because of the lowering voltage on 9|,producing a chart of uniform character. The total time over which theautomatic volume control functions is controlled by resistor 14 aspreviously mentioned. The size of the middle portion of the A. V. C.discharge curve is largely controlled by resistor 73. The size thereofimmediately after the automatic volume control has functioned iscontrolled by resistor 12 while resistor 18 controls the size which theoutput signal must reach for the A. V. C. to function at all. Thus theoperator can, by appropriate settings of the various variables obtainthe type of chart trace most suitable for the terrain being explored andfor the use desired to be made thereof. By making trial charts he cancompensate for various unexpected factors and achieve superior results,obtaining a chart useful throughout its full range.

As has been pointed out hereinbefore, it is highly desirable that thegain control be selectively responsive to the arrival of waves ofdifferent frequencies so as to suppress to.- a greater extent waves ofhigh amplitude and high frequency than waves of comparable amplitude oflow frequency. To this end I may employ the arrangement illustrated moreparticularly in Figure 1A of the drawings in which, as hereinbeforepointed out, a requencyresponsive filter, comprising, for example,resistance 65 and condenser 66, is associated with the source of supplyto the automatic volume control tube. By proper selection of the valuesof these filter elements, signals of high frequency may be favored andgreater reduction of the gain may thereby be obtained whenever thehigher frequencies are being received. For example, satisfactory resultsmay be achieved if the impedance of the condenser 66 at 50 cycles issubstantially equal to the impedance of winding 58 of transformer 59which is comparable to the value of resistance 65. If the highest usablefrequency is higher than 50 cycles it may be desirable for thisrelationship to exist at this higher frequency.

Under these conditions the threshold value for the automatic gaincontrol is in effect substantially higher for signals in which lowfrequencies predominate than for signals of comparable amplitude inwhich high frequencie predominate. Thus whenever high frequenciespredominate, and the signal peaks on separate cycles of the recordedtrace are therefore quite close, the sensitivity of the amplifier isreduced on the occurrence of relatively small increase in signalamplitude, so as to minimize confusion on the record. When, however, thesignal frequencies are predominantly low, and the peaks are fairlywidely separated, amplification is not closely controlled by theautomatic gain circuit, the gain is higher for signals of comparableamplitude, and the clarity of the record is improved without anyapparent crowding of the successive peaks.

It will be appreciated that the filtering action just described may beobtained in some other point in the circuit. For example, a suitablefilter might be associated with the secondary winding 62 of the outputtransformer with similar results.

Having thus described the invention, what is claimed as new and desiredto be secured by Letters Patent is:

1. In apparatus for use in seismic surveying, the combination with aseismometer for converting incoming seismic energy into electricalwaveform signals, and means for amplifying and recording such signals,said amplifying means including at least one thermionic valve amplifier,of a thermionic gain control valve for applying to a control grid ofsaid valve amplifier a more negative bias in response to increase inamplitude of incoming energy above a predetermined amplitude, meansinitially biasing the grid of said gain control valve beyond cut-off,and means applying to the grid of said gain control valve a voltagederived from the incoming energy such as to render the grid potentialless negative than said initial bias, said last named means including afrequency selective device favoring the higher frequencies within ausable band of frequencies.

2. In apparatus for use in seismic surveying, the combination with aseismometer for converting incoming seismic energy into electricalwaveform signals, and means for amplifying and recording such signals,said amplifying means including at least one thermionic Valve amplifier,Of a thermionic gain control valve rectifier for applyingto a controlgrid of said valve amplifier a more negative bias in response toincrease in amplitude of incoming energy above a predeterminedamplitude, means initially biasing an element of said gain control valverectifier beyond cut-01f, and means applying to an element of said gaincontrol valve a voltage derived from the incoming energy such as torender the element potential less negative than said initial bias, saidlast named means including a filter for emphasizing energy of higherfrequency within the range of useful frequencies, whereby the voltage ofthe gain control valve is varied to a greater degree by incoming energyof predominantly high frequencies than by incoming energy ofpredominantly low frequencies of comparable amplitude.

3. In apparatus for use in seismic surveying, the combination with aseismometer for converting incoming seismic energy into electricalWaveform signals, and means for amplifying and recording such signals,said amplifying means including at least one thermionic valve amplifier,of a thermionic gain control valve for applying to a control grid ofsaid valve amplifier a more negative bias in response to increase inamplitude of incoming energy above a predetermined amplitude, meansinitially biasing the grid of said gain control valve beyond cut-off,and means responsivein greater degree to high than to low frequencieswithin the usable frequency range for applying to the grid of said gaincontrol valve a voltage derived from the incoming energy such as torender the grid potential less negative than said initial bias.

4. In apparatus for use in seismic surveying, the combination with aseismometer for converting incoming seismic energy into electricalwaveform signals, and means for amplifying and recording such signals,of means operable in response to increase in signal energy above apredetermined ampliutde for decreasing the gain of said amplifyingmeans, said last named means including a device for initiating operationof said means to decrease the gain, said device affordin filteringaction favoring the higher frequencies within the usable range, wherebythe gain at all frequencies is decreased in greater degree by si g nalsof large amplitude and predominantly high frequency than by signals ofcomparable amplitude and predominantly low frequency.

OLIVE S. PETTY.

